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Multiple small cables in lieu of one large?

AsyDRabbit

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Joined
Aug 30, 2020
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5
Hello all,

Plaintive pommy plea for ... pelp?

In the manual for an inverter charger I read:

When ordering the wire using one cable is always best. However, you can add up smaller cable’s mm
sq to equal the equivalent larger size. For example, if you want 100mm sq using 2x50mm sq would be
ample, likewise with 3x 33mm sq etc.. Do not forget that each cable shall need fusing at half (or a third
etc) of the single cable rating, its not safe to put multiple cables on a single fuse .


How would this be managed?
I'm seeking to connect a 2,000W 12V inverter charger to a parallel pairing of 300aH 12V LiFePo4 batteries.

I've worked out that a 70mm² wire would be required to run into my supply busbar, far bigger than my current largest size of 25mm². Conveniently for price as well as having to resize my earthing and other linked battery cables, based on the quote above I could use the excess of my 25mm² to connect the inverter, but I don't understand how it would be connected into the inverter charger.

Simply three separate fused cables to place onto the unit terminals?

The inverter charger can be found here:

And the batteries here:
https://www.qualitysource.co.uk/col...60ah-continuous-bms-lithium-battery-12v-300ah

I'm open to switching out the inverter charger if someone has a more viable suggestion. I know it's not meant for use with LiFePo4, but I figure with the appropriate specific parameters set for the charging profile it should be fine.
 
2000 watts * 1.15 inverter efficiency / 12 volts * 1.25 fuse headroom = 239.583333333 fuse amps
2x 25mm2 will give you some headroom.
Use this on the battery side with the https://www.waytekwire.com/item/45569/EATON-s-Bussmann-Series-MRBF-Mounting-Bar-3-8-/
With 2 of these https://www.waytekwire.com/products/1367/Fuses/&Fuse-Type=MRBF&Current-Rating=125A
You can stack the lugs on the non fused side or terminate on a busbar.

If you want to go 3x you can use this https://www.waytekwire.com/item/78261B/Blue-Sea-Systems-5196B-MRBF-Surface-Mount/
 
I had to read much of the NEC book for a project awhile back and I recall it states in residential you can't use multiple conductors to carry current but you can in industrial applications. But do what you want. I wouldn't do it myself because it will look likely shobby work and the fact that getting the right wire isn't that hard. Extra fuses and holders will likely cost more in the long run and I do see a fuse tripping and then all of them tripping, one connection coming loose which is very common will cause this.

In industrial settings I think they let you do it because really big wire is so hard to work with.

Good luck.
 
Many thanks both ❤️

The concept made sense to me but as I have no actual electrical education I'm wary of just how far "common sense" should be trusted ?

Plus it just seemed a grossly inefficient way of doing it ?

The extension blocks you posted would definitely keep things neat though.

Are inverters not meant to be paired up with T-class fuses? Do cube fuses do the same job?

My original plan is that I have separate supply and load busbars. These have the space for me to mount the fuses directly onto then after a little finagling with the lids. Three +/- wires from the inverter into the fuses onto the load busbar.



Those bars are only for 150A, but I could up that to 300A if the principle is sound - the only other loads currently planned to be connected are the 12V DC side, behind a 60A fuse.

It makes sense in my head that this should work, but then it also makes sense to me that chocolate chips work well in the base of a four cheese pizza so it's best to check, right? ?
 
I had to read much of the NEC book for a project awhile back and I recall it states in residential you can't use multiple conductors to carry current but you can in industrial applications. But do what you want. I wouldn't do it myself because it will look likely shobby work and the fact that getting the right wire isn't that hard. Extra fuses and holders will likely cost more in the long run and I do see a fuse tripping and then all of them tripping, one connection coming loose which is very common will cause this.

In industrial settings I think they let you do it because really big wire is so hard to work with.

Good luck.


Yeah, the multiple wires aspect has me a bit perturbed, although I'm unsure why - again, my education is largely from the gregarious Greg V and prodigious Mr Prowse more than anything else.

I'm considering just lowering the inverter rate. I only really need triple figures at present, I'm just future proofing myself, but it wouldn't be that hard to adjust the main battery and inverter wiring in the future as they're all sited together in one accessible area.

I'm something of a do it once, do it right person, which is why I'm leaning towards doing it now though ??‍♂️
 
I had to read much of the NEC book for a project awhile back and I recall it states in residential you can't use multiple conductors to carry current but you can in industrial applications.

NEC may not strictly apply to @AsyDRabbit as he self identifies as a "pommy".
I do agree that 1 wire is generally preferable to >1.
 
It is not a recommended practice to use multiple wires instead of one cable. But if you do, fuse each wire for its capacity and not for the total circuit amperes. Example; proper 2/0 cable fuse at 250 or 300 amps for inverter. four separate 6 awg fuse each at 100 amps.
 
Are inverters not meant to be paired up with T-class fuses? Do cube fuses do the same job?

My original plan is that I have separate supply and load busbars. These have the space for me to mount the fuses directly onto then after a little finagling with the lids. Three +/- wires from the inverter into the fuses onto the load busbar.



Those bars are only for 150A, but I could up that to 300A if the principle is sound - the only other loads currently planned to be connected are the 12V DC side, behind a 60A fuse.

I wonder how performance of a 3 GBP cube fuse compares to a 23 GBP class T fuse?
The better fuses have interrupting specs, like "Breakthrough Current 20,000 A @ 160 V DC" for the one I linked below.
What is the short-circuit current capability of your battery?

your fuse has a spec for VAC but not VDC. It may not be effective interrupting DC.
Here's one that is (link may not be accessible from outside US, at least not unless you create a log-in.)


Of course you could used two 150A busbars each with no more than 150A of fused circuits.

"I'm seeking to connect a 2,000W 12V inverter charger to a parallel pairing of 300aH 12V LiFePo4 batteries."
Each battery could have its on smaller wire and fuse, so inverter gets sum of both. But maybe you want to parallel all cells so a single BMS handles all?

"I'm open to switching out the inverter charger if someone has a more viable suggestion."
How about connecting batteries in series for a 24V inverter? (or 48V)
 
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